The present invention relates to an improvement in a video signal transmission system used when a video signal is transmitted through a signal line and a corresponding image is displayed.
An information processing apparatus is normally used in three ways of usage. In the first usage, an input device and a CRT display are integrally attached to an apparatus body, and a signal is appropriately processed by the apparatus body based on information from the input device to be displayed on the CRT display. In the second usage, information transmitted from a remote place is appropriately processed by the apparatus body to be displayed on the CRT display. In the third usage, information from an input device located relatively close the apparatus body is appropriately processed by the apparatus body to be displayed on the CRT display.
The system of the present invention is mainly applied to the latter two usages in view of information transmission using the signal lines. In this case, a video signal (e.g., red, green, and blue color signals) consisting of a plurality of bits and a 2-bit sync signal (horizontal sync signal and vertical sync signal) must be transmitted using external signal lines, and a video signal must be displayed on the CRT display of the apparatus body based on these signals.
In general, timings of various signals and a CRT screen have the relationship shown in FIG. 1. More specifically, on a CRT screen, a central portion serves as display area 1, and horizontal back porch 2 and horizontal front porch 3 are assigned to the two side portions of display area 1. In addition, vertical back porch 4 and vertical front porch 5 are assigned to portions above and below display area 1. H. sync signal 6, V. sync signal 7, and video signals 8 and 9 are supplied at timings shown in FIG. 1.
The sync signals and the video signals have the timing relationship shown in FIG. 2. More specifically, H. sync signal 6a for each horizontal scanning line is sequentially generated in response to the leading edge of V. sync signal 7a at equal intervals. In FIG. 2, V. sync signal 7b and H. sync signal 6b are enlarged V. sync and H. sync signals 7a and 6a. H. sync signal 6b is supplied to the CRT display during a predetermined H. sync period from the leading edge of V. sync signal 7b. H. back porch 2, display period llb of video signal 11a, and H. front porch 3 are set between two adjacent H. sync signals (6b--6b) having H. sync periods (6c).
The CRT display normally reproduces and displays an image once per 20 to 25 msec at a frequency of 40 to 50 Hz. When an image is reproduced at a V. sync frequency lower than 40 Hz, flickering occurs on the screen. The screen is constituted by 640 dots in the horizontal direction, and 500 dots in the vertical direction in the case of a personal computer. The screen may be constituted by 1024 dots in both the horizontal and vertical directions to have a high resolution, if necessary. In this case, a display period per dot corresponds to about 40 to 45 msec.
When various signals (6 to 9) are transmitted to the CRT display with the above screen configuration using a cable, the contents of original signals may be lost due to attenuation of the signals and a decrease in a slew rate of the signals unless an inter-line capacitance, a line resistance, and the like of the cable is reduced as low as possible.
In an information processing apparatus employing the above video signal transmission system, the inter-line capacitance and the line resistance of the cable are increased along with the higher resolution image configuration, as a distance between transmitter and receiver systems of the video signal is increased. Thus, a transmitted image cannot be normally displayed on the CRT display due to waveform distortion of and/or attenuation of the transmission signal.
It may also be proposed to use an optical fiber in place of a normal cable without extending the cable, and modulate/demodulate signals by a Manchester code. With this system, a cable length can be increased to several km. However, the number of cores of the cable, corresponding to that of video signals and sync signals, is required, resulting in an expensive system. Therefore, it is a material problem to reduce the number of cores of the cable. Even if no optical fiber is used or even if the cable is not extended to several km, a reduction in number of cores of the cable is still very effective to improve product value (or to reduce a manufacturing cost).